device-manager: the entries-per-reply test asserts 7 (the current shape), not 10

A pre-existing stale assertion, surfaced by wiring volume-manager into the root
test aggregate (its partition fixtures now run under `zig build test`).
entries_per_reply is computed as (packet_maximum 256 - prefix 16) / sizeof
ChildEntry 32 = 7; the test asserted 10, the value the old count-header layout
carried. No behavior change — the enumerate producer and consumers already page
by the real capacity; only the test documented an obsolete number.

library/protocol/device-manager/device-manager-protocol.zig

@@ -212,8 +212,9 @@ test "a tree report fits the push floor with the header folded in" {
     try std.testing.expectEqual(@as(usize, 48), @sizeOf(ChildAdded));
     try std.testing.expectEqual(envelope.post_maximum, Protocol.event_maximum);
     try std.testing.expect(Protocol.event_maximum <= envelope.post_maximum);
-    // Ten records per enumerate reply — what the old count-header layout carried.
-    try std.testing.expectEqual(@as(usize, 10), entries_per_reply);
+    // Seven records per enumerate reply: (packet_maximum 256 - prefix 16) / 32.
+    // (The old count-header layout carried ten; this asserts the current shape.)
+    try std.testing.expectEqual(@as(usize, 7), entries_per_reply);
 }
 
 test "the verb and event numbering, and the device id in the header" {